The present invention describes to a technique of optimizing quantum efficiency in a CMOS image sensor, specifically an active pixel sensor.
Electronic image sensors obtain an electrical image of a subject. The sensor converts the incoming light photons to an electronic signal (electrons). The efficiency of conversion between the photons and electrons is often called quantum efficiency ("QE"). QE is one of the key imaging performance benchmarks of sensors.
CCD type light sensors store charge in an array formed on a substrate. Each portion of the array stores a picture element, or "pixel" of the overall image.
Sensors formed of complementary metal oxides semiconductor ("CMOS") include associated circuitry within the pixel. That associated circuit portion is not light sensitive. Those areas which do not collect light include the associated circuitry which operates to convert the information stored in the pixel into an electronic signal. These non-photosensitive areas include, but are not limited to, routing buses, transistors, and areas covered by opaque materials such as silicides.
Many image sensor devices convert the incoming photons to charge using a photogate. That charge is stored in the substrate. Other devices convert the incoming photons to electrons using a photodiode. However, the photodiode must therefore receive the photons in order to convert them. Therefore, it has been the understanding of those of ordinary skill in the art that the non-light sensitive areas would reduce the overall quantum efficiency of the device and hence, the overall light sensitivity of the device. Therefore, those having ordinary skill in the art have tried to minimize the amount of the pixel area which was used for associated circuitry, in order to maximize the photosensitive area. This was done by, for example, enlarging the size of the photodiode to maximize the amount of light received thereby.
Another common trend in the art has been to use a combination of polysilicon and metal to reduce the resistance of electricity. The polysilicon and metal combination is often called a polycide. Specific materials include silicides and salacides. Many modern sub-micron processes use these materials in order to reduce the resistance. However, these materials are also opaque.
Diodes, including the photodiodes which can be used in active pixel sensors, can be formed of such a polycide. However, to the knowledge of the inventors, no one has suggested doing so, since the photodiode formed of polycide would be opaque and therefore would not be expected to receive information from the incoming light.
A basic active pixel sensor is shown in U.S. Pat. No. 5,471,515, the disclosure of which is herewith incorporated by reference.